This invention relates to reducing electrical signal interference which arises due to parallel contacts in electrical connectors. More particularly it relates to the partial cancellation of pair to pair interference or cross talk induced into a signal carrying cable pair by FCC type modular jacks and plugs, or other input/output signal connectors.
The Federal Communications Commission has adopted certain architectural standards with respect to electrical connectors utilized in the telecommunication industry so as to provide intermatability. The connectors most commonly utilized are FCC type modular plugs and jacks. The plug is commonly terminated to a plurality of wires which may be connected to a telephone handset or other communications device. The corresponding jack is commonly mounted to a panel or a printed circuit board which in turn is connected to a telecommunications network. A typical FCC jack is described in U.S. Pat. No. 4,648,478.
Often a plurality of jacks, such as six, are placed adjacent to one another in a housing with the printed circuit board forming the backside of the housing. Each jack includes a plurality of elongated contacts which are closely spaced and parallel to one another. A typical jack having four adjacent and parallel contacts is shown in FIG. 1. Jack 10 includes electrical contacts 12 which are housed in the hollow portion of the jack and include two pairs of parallel contacts. Each pair 14 and 20 form separate communication circuits, each of which will be referred to herein sometimes as a signal pair. Pair 14 includes conductor 16 and conductor 18 and pair 20 includes conductor 22 and conductor 24. Each of these contacts extend through the backside 26 of jack 10 and are soldered to circuit board 28. The lengths of the contacts running through the jack are predetermined by connector design considerations.
When an electrical signal of a given frequency is applied to a pair of conductors, unequal portions of signal energy is transmitted to the individual conductors of an adjacent pair by each conductor of the signal pair. This coupling is primarily due to capacitive and inductive coupling between adjacent conductors being substantially higher than the couplings of the other conductor of the signal pair resulting in a phenomenon known as cross talk. This phenomena is further exaggerated when both conductors of the signal pair are placed adjacent to and outside of opposing conductors of the other signal pair. For ease of discussion adjacent conductors which are not a signal pair in themselves are referred to herein as a cross-talking pair. For example conductors 16 and 22 form a cross-talking pair. The extent of the cross talk is governed by such parameters as the space between the adjacent conductors, the dielectric constant of the matter between such conductors and the distance in which such conductors are closely spaced and parallel to one another. Furthermore, the extent of the cross talk is a function of the frequency of the signal on either or both of the signal pairs. Cross talk increases logarithmically as frequency of the signal increases and is commonly expressed as 10.times.log of the ratio of the cross talk energy divided by the signal energy (decibels or dB).
As FCC modular plugs and jacks are utilized more and more in high frequency data and communication applications, cross talk which arises in the adjacent and parallel contacts of the jack, and to a lesser degree in the plug, has become a problem in the industry. Furthermore, because of miniaturization as well as FCC rules, these jacks are quite small resulting in a very close spacing of the contacts which exacerbates the problem.
A current industry standard permits no more than -46 dB of near end cross talk at 16 Megahertz (MHz). Currently it is anticipated that the standard for near end cross talk will be shortly reduced to less than -56 dB. Using good prior art design techniques, such as that shown in FIGS. 3 and 4, one is able to reach -50 dB of cross talk at 16 MHz utilizing commonly available modular plug and jack combination. The current thinking is that the routings of the individual circuit paths for each signal pair on the circuit board should generally be parallel to each other and the signal pairs should be separated from one another as much as possible and be non-parallel. That type of design technique on the circuit board does nothing to reduce the cross talk which arises within the connector which is attached to the circuit board but simply, at best, maintains the status quo.